Adam: Yeah, I use TinyPng too. It's very nice. I'm really thinking about general guidelines, especially for banner dimensions. Do you generally recommend something like 1600 x 900 pixels? Smaller, larger? Then from there I can sort out compression and getting file sizes the right amount. But I'm less clear about what kind of banner size I'm shooting for. Oddly enough gallery photos are easier to plan for as they really don't need to be as big.
It is going to vary from project to project and image to image as well as how you're using the image. Have you set a max-width, is the image allowed to go all the way to the edge of the browser at all times, etc. One of the wondrous things about developing websites is the experimentation. Try different sizes, compression methods, etc. and see what the outcomes are.